1. Technical Field
The present invention relates to an overcurrent protection circuit and a voltage regulator incorporating the same, and more particularly, to an overcurrent protection circuit that prevents excessive current in a constant voltage regulator supplying constant power to electronic equipment, and a voltage regulator incorporating such an overcurrent protection circuit.
2. Discussion of the Background
Overcurrent protection circuits are employed in power supplies to protect electronic components from excessive current. One typical application of overcurrent protection is in a constant voltage regulator, which limits current flow in an active component and load circuitry used therewith.
FIG. 1 is a circuit diagram illustrating a constant voltage regulator 100 incorporating a conventional overcurrent protection circuit.
As shown in FIG. 1, the voltage regulator 100 includes a main circuit formed of a P-channel metal-oxide-semiconductor (PMOS) transistor P101, resistors R101 and R102, a reference voltage generator 101, and an error amplifier 102, as well as an overcurrent protection circuit formed of PMOS transistors P102 and P103, a resistor R103, and N-channel metal-oxide-semiconductor (NMOS) transistors N101 through N104.
Basically, the voltage regulator 100 is a series regulator that regulates a voltage Vin input to an input terminal IN to output a given constant voltage Vout to an output terminal OUT connected to a load circuit, with the overcurrent protection circuit serving to prevent excessive current flow in the output transistor P101 and the load circuit.
In voltage regulation, the resistors R101 and R102 generate a feedback signal Vfb by dividing the output voltage Vout, while the reference voltage generator 101 generates a reference voltage Vref. The error amplifier 102 compares the voltages Vfb and Vref to generate a control signal that drives the gate of the transistor P101. According to the control signal, the output transistor P101 outputs the constant voltage Vout, while passing therethrough a current i101 to output a current iout to the output terminal OUT.
In the overcurrent protection circuit, the transistor P103, having its gate connected to the gate of the transistor P101, conducts a current i102 proportional to the current i101. The transistors N101 through N103 form a current mirror to generate a current i103 that is proportional to the current i102, and therefore, to the current i101 as well.
The current i103 thus generated flows through the resistor R103 to generate a voltage drop thereacross, equal to the product of the current i103 and a given resistance r103 of the resistor R103 according to Ohm's law. As the current i103 varies in proportion to the output current i101, the voltage drop across the resistor R103 drives the gate of the transistor P102, which, having its drain connected to the gate of the output transistor P101, turns off the output transistor P101 upon an overcurrent condition in which the current i101 exceeds a given current limit.
Such overcurrent occurrence and subsequent current limitation is accompanied by a reduction in the output voltage Vout. When the output voltage Vout falls below a given threshold, the transistor N104, having its gate connected to the output terminal OUT, its drain connected to the source of the transistor N103, and its source connected to ground, turns off, thus changing the ratio between the proportional currents i102 and i103.
More specifically, given that the NMOS transistors N101, N102, and N103 have sizes or channel width-to-length ratios n101, n102, and n103, respectively, the ratio of the current i102 to the current i103 is (n101+n103):n102 when the transistor N104 is conductive, and n101:n102 when the transistor N104 is nonconductive. Thus, in response to the output voltage Vout falling below the threshold voltage, the transistor N104 turns off to sharply reduce the current limit to n101/(n101+n103) times its original value.
Such current limit immediately switched according to the output voltage Vout results in the current i101 being maintained substantially constant regardless of whether the load is shorted or partially shorted. Such current limitation is also seen in certain constant power supplies incorporating a foldback current limiter, another typical form of overcurrent protection circuit. For example, there is a constant power supply with a foldback current limiter featuring low power dissipation regardless of whether the load is shorted or partially shorted.
One drawback of the technique depicted in FIG. 1 is that a system or load deriving power from the voltage regulator is not informed of operating status of the overcurrent protection circuit. In particular, monitoring current limitation where the output voltage changes with the output current is difficult, since the current limit can oscillate as the output voltage rapidly changes in response to changes in the limited output current. Such failure to relay and monitor the operating status of the overcurrent protection circuit makes it difficult to diagnose malfunctions in the system powered by the overcurrent-protected voltage regulator.